This invention relates to rotary joints for conducting high frequency electrical energy from one transmission line to another while allowing one to rotate with respect to the other.
A widely used rotary joint structure for conducting high frequency electric waves from one transmission line to another is most often a coaxial transmission line in two sections that match each other mechanically and electrically where they meet so that one section can rotate on their common axis with respect to the other one. The mechanical and electrical matching is readily accomplished using the coaxial type transmission line, because the line is essentially a figure of revolution and since one section must revolve on the common axis with respect to the other section, there is no change in the mechanical or electrical matching as one section revolves with respect to the other. For these reasons, it has been the practice to make the mating rotating parts of a transmission line rotary joint in the form of a coaxial line even though the input and output transmission lines may be waveguide or other types of transmission line. It then becomes necessary to match the input and output transmission lines to that coaxial line section to insure proper performance as measured by insertion loss and standing wave voltage ratio (VSWR) in the system. Heretofore, this matching from a waveguide section to the rotary coaxial line section has been done at an L turn, and so the axis of the waveguide is not common with the axis of the coaxial line rotary joint. It is one object of the present invention to provide a rotary joint for input and output waveguides on a common axis with the coaxial rotary line so that a straight transmission line is defined by the input waveguide, the coaxial line rotary joint, and the output waveguide.
In a rotary joint transmission line where, as usual, the abutting rotary parts define a coaxial transmission line and other types of transmission lines such as waveguides connect as the input and output to opposite ends of the rotary coaxial line, it is often preferred that the dielectric between the conductors of all these transmission lines be air. There are many reasons for this, one being that air does not degrade or change with time and it is intimately contiguous with all the conductors. Also, where a solid dielectric interfaces with another or interfaces with air, undesired reflections can occur. However, with air as the dielectric, matching a waveguide to a coaxial line is commonly done at an L section transformer so that the input and output waveguides are not coaxial with each other or with the rotary joint. This is particularly the case where the waveguide is ridged. Accordingly, it another object of the present invention to provide a rotary joint for connecting input and output waveguides on a common axis with the rotary coaxial section, wherein the dielectric throughout is air.